Product datasheets from different manufacturers often display various "pressure dew point" values for refrigerated dryers: 0°C, 1°C, 1.6°C, 1.7°C, 2°C, 3°C, 2-10°C, 10°C, and so on. This makes choosing a refrigerated dryer inconvenient, so it's important to thoroughly discuss the maximum pressure dew point a refrigerated dryer can achieve.
We know that the pressure dew point of a refrigerated dryer is actually limited by three factors:
1. The evaporation temperature freezing point;
2. The evaporator's heat exchange area cannot be increased indefinitely;
3. The separation efficiency of the "gas-water separator" cannot reach 100%.
It's normal for the final cooling temperature of the compressed air in the evaporator to be 3-5°C higher than the refrigerant's evaporation temperature. Excessively lowering the evaporation temperature is also ineffective. A small amount of condensed water, limited by the efficiency of the gas-water separator, can be converted to water vapor during heat exchange, increasing the moisture content of the compressed air.
All these factors combined make it difficult to keep the dryer's pressure dew point below 2°C. As for labels like 0°C, 1°C, 1.6°C, and 1.7°C, they're often more commercial than practical.
In fact, setting a dryer's pressure dew point below 10°C is no longer a low standard for manufacturers. The Ministry of Machinery's standard JB/JQ209010-88, "Technical Specifications for Compressed Air Freeze Dryers," stipulates a pressure dew point of 10°C for freeze dryers; while the national recommended standard GB/T12919-91, "Marine Controlled Air Source Purification Devices," requires a dryer's atmospheric pressure dew point between -17°C and -25°C, equivalent to 2°C to 10°C at 0.7 MPa.
Most domestic manufacturers specify a range for dryer pressure dew points. The lower limit specifies the minimum load required to prevent ice from forming inside the dryer, while the upper limit specifies the moisture content the dryer should achieve under rated operating conditions. Therefore, this can be regarded as a rigorous annotation method.